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The
Blue 600W - SN-600AS
The
600W model in shiny blue, with a
quirky 14cm fan, provides 33A on
the 12V rail. There are
550,500,450 and 400W models
according to the label, all with
diminishing current on the 12V
and 5V rails.
 [June 2021] When
I plugged in one of my spare
computers and it went "pop", I
immediately knew the Power
Supply Unit had failed. It had
been problematic some months ago
when it could no longer cope
with a graphics card, even
though it was rated at 600W (it
was some years old though).
I'd
inspected the components prior
to the failure and
noticed some capacitors were
failing.
At
that time I didn't even
entertain the idea of meddling
with a power supply so I just
swapped that unit into a less
demanding computer where it
worked ok. That was until it
went pop!
| Power supplies deal with high voltages and
should not be meddled with, even when unplugged the
components can still hold a dangerous charge. I recently got
a lesson in this when I got a shock off an iMac's screen
inverted board [see here]. |
I
reopened the 600W PSU (after
leaving it for a day to
discharge - although this is no
guarantee that the capacitors
don't still hold a charge). In
addition to the obvious
capacitor issue there was fresh
new damage from the component
that had seemingly gone pop.
I
wasn't sure what type of
component this was, let alone
its value. It looked like the
damage to it could have
obliterated any details on it,
but fortunately it was one of a
pair so I proceeded to remove
both of them.
It
turned out I could read the
details on both of them: "SCK
053". While that was too vague
for me I was able to find a data
sheet online that revealed that
SCK was a product code for an "NTC-Surge
Current Killer" or Thermistor,
and 053 indicated 5 ohms / 3A. I
found I could order exact
replacements, or spend a little
less and perhaps get them a
little sooner (still coming from
China however) if I bought a set
of different ones but with the
same rating (I opted for the
latter).
I
tested the "thermistor" that
appeared undamaged but the
reading appeared haphazard with
the resistance either being
different each time and not
matching 5 ohms, or not
registering at all. Perhaps that
was the nature of these
components, or both were
damaged.
|
Thermistors change resistance
with temperature changes; they
are temperature-dependent
resistors. They're perfectly
suited to scenarios where one
specific temperature needs to be
maintained, they're sensitive to
small changes in temperature...
|
That
would perhaps explain the
variable reading, but the
conclusion was that I couldn't
really test the "probably ok"
one without a known good one to
compare with, but I'll likely
replace them both anyway.
I
also removed the obviously
failing capacitors from the power supply making a note
of their values, and
orientations; unlike the
motherboard I'd recapped
recently, power supply boards
don't always have
the capacitor orientation
indicated on them, and this is
important [or it can also be
wrong]. I'm not sure if the thermistors have an orientation
but I have noted that also just
in case.
When
the replacement thermistors
arrived I soldered them in
place, reassembled the PSU, and
gingerly tested it... It worked.
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